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When I raise my arm out to the side, I feel a grinding sensation under my shoulder blade. Sometimes it snaps loud enough for my wife to hear. It doesn't hurt but I wonder what it is.

You wouldn't know it without being told, but raising the arm overhead requires complex coordinated interactions of the clavicle (collar bone), glenohumeral (shoulder) joint, and scapula. The entire scapulohumeral structure is held together by muscles, tendons, and ligaments. Protective pads called bursa (bursae when referring to more than one) make it possible for smooth, gliding movements of all the parts. Sometimes people (especially athletes) develop grinding, snapping, crunching, or popping called crepitus that can be felt by the affected person and even heard by others. When crepitus affects the scapula as it moves over the rib cage (thorax), it is referred to as scapulothoracic crepitus or snapping scapula. What causes scapulothoracic bursitis or crepitus? Normal variations in the shape, size, and position of the scapula can cause muscle and tendon to rub the wrong way. There are those protective pads (bursae) to help ease the tension between muscle and bone or even muscle and muscle. But if there's a bony bump on the scapula or an extra long end of the bone, the extra wear and tear of muscle/tendon flipping back and forth over the prominence can result in crepitus. Now, crepitus isn't always a problem. It turns out that one-third of all adults notice some snapping, grinding, or popping as they move their arm. It never results in painful symptoms or loss of function. What we are talking about here are those people who either suffer some injury to the arm (trauma) or overuse the arm (repetitive irritation of the bursa). Either one can cause inflammation of the bursa (bursitis) and eventually crepitus from scarring and fibrosis. In a smaller number of people, there are other potential causes of scapulothoracic bursitis and crepitus besides trauma and overuse. For example, bone tumors or bone spurs may result in a reactive bursa. A bursa forms where no bursa normally exists. Muscle tears, muscle atrophy, structural spinal deformities like scoliosis (curvature of the spine), and rib or scapula fractures that don't heal properly can also create these types of problems. It may be a good idea to have your physician take a look and make sure there isn't a serious cause of this problem. If there's nothing seriously wrong requiring surgical treatment, you may benefit from a program of specific exercises to balance the muscular action that helps move the scapula and shoulder. A physical therapist can evaluate you and give you a program to follow at home with some supervision. This type of rehab may save you some trouble later if it is addressed early on.

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